I met up with my boss (GM of the company)in 2012 to discuss on the possibility of increasing my salary due to the increase in responsibilities. Thereafter, we had a discussion and my boss presented to me a 3-year career plan whereby my salary will be increased RM300 for 3 years starting immediately. I accepted his proposal.

However, the letter received from the HR dept only indicated the increase for the 1st year but the subsequent increases were not included. I then emailed to the HR Manager and copy to my boss requesting him to revise the letter and in my email, I detailed out the increases as discussed with my boss earlier. However, the HR never revise the letter and also did not reply to the email. Nevertheless, I still get my 1st increase.

Come 2013, when I'm supposed to get the 2nd increase as discussed, I approached my boss to remind him. We underwent a performance appraisal session , which is a normal HR practise before the increase takes place officially. However, the Finance dept is arguing the increase because there was no official documentation to proof that the 2nd and 3rd increase were discussed and approved to proceed. Since my boss is leaving end of next month, they were saying that my boss and I are trying to conspire to push it through as if the increases were only discussed now when it indeed took place more than a year ago!

My question:
Is there any way that I can fight for the increases as it was promised to me and due to that, I stayed with the company.

Well, a bit of tricky situation. But my advice would be for you to hold to your position, ie, that increment was approved and get your boss to confirm it in writing. The only problem would be what if they still refuse to pay. Then you got a real tricky situation since your only option might be constructive dismissal which I don't think you would want to consider considering that i believe you still want the job.